The present invention relates to a method for transmitting a digital audio signal via an expansion card connector as set forth in the introductory part of the appended claim 1, an expansion card as set forth in the introductory part of the appended claim 8, as well as a mobile station as set forth in the introductory part of the appended claim 15.
In digital mobile communication networks, the audio signal is transmitted in digital form, whereby an analog audio signal must be converted into digital form before transferring it to the mobile communication network. In a corresponding manner, in a mobile station at the receiving stage, the digital audio signal is converted into an analog audio signal e.g. for conducting it to a loudspeaker. For example in a GSM mobile station, pulse code modulation (PCM) can be used in the analog/digital conversion. In addition to analog to digital and digital to analog conversions, also signal filtering, encryption and other measures are taken, if necessary, in a known manner, but there is no need to describe them in detail in this context.
In current digital mobile stations, the processing of audio signals takes place largely in the digital signal processing unit DSP. The analog to digital and digital to analog conversions of the audio signal take place in a codec, advantageously a PCM codec. In conventional digital mobile stations, the serial input/output port (SIO) of the digital signal processing unit is connected directly to the codec. In mobile stations, audio signals are transmitted in digital form primarily within the digital signal processing unit as well as between the digital signal processing unit and the codec.
Particularly in connection with portable data processors (PC), mobile station applications have been developed where at least the transmitter/receiver unit TX/RX of the mobile station is arranged in card form in accordance with the PCMCIA standard. This kind of an extension card can be advantageously used e.g. for transmission of data between data processors via a mobile communication network. Thus, the expansion card is used as a modem for the data processor. However, expansion cards of this kind are also equipped with properties for transferring an audio signal e.g. for setting up an ordinary voice connection via the mobile communication network. In expansion cards of prior art, the audio properties are implemented in a way that the expansion card is provided with a codec for conducting the analog-to-digital and digital-to-analog conversions required for the digital audio signal. Further, the expansion card is provided with an audio connector or the like e.g. for connecting an earpiece and a microphone with the codec. However, this solution has the disadvantage that the codec and the connector raise the manufacturing costs of the expansion card and require space on the expansion card.
In the international standard TIA PN-3131: "Voice Control Standard for Asynchronous DCE", it is presented that in expansion cards, the audio signal should be transmitted through the expansion card connector via the same serial bus than the control signals (control data). On the other hand, this standard supports only the telephone answering function, wherein it is necessary to transfer audio signals simultaneously in only one direction at a time, i.e. a half duplex mode, because the standard assumes that an expansion card like this has a separate external audio connector. However, telephones usually require simultaneous transmission of audio signals in both directions, i.e. a full duplex mode. It has been found in practice that the transfer capacity of this serial bus is not sufficient for transmitting both control signals and full duplex voice signals.